School authorities in Wisconsin, US said a gunman was “neutralised” outside a school in Mount Horeb on Wednesday, and no one inside the building was injured.

Authorities said without giving details that the “alleged assailant” was harmed, and a witness said she had heard gunshots and seen children running.

Photos by the Wisconsin State Journal showed more than a dozen police vehicles including SWAT-style trucks near the school with their emergency lights flashing.

The district said in several posts on Facebook beginning around 11.30am that students at all the district’s schools were on lockdown and family members were told not to come to any schools.

“An initial search of the middle school has not yielded additional suspects,” a post around noon said. “As importantly, we have no reports of individuals being harmed, with the exception of the alleged assailant.”

In an earlier post, the district said “the threat has been neutralised outside of the building”.

The posts by the district did not provide any details about what had happened.

Jeanne Keller was in her shop The Quilting Jeanne located close to the campus that includes the middle school and another school building when she heard about five gunshots.

“It was maybe like pow-pow-pow-pow,” Ms Keller told The Associated Press by phone. “I thought it was fireworks. I went outside and saw all the children running … I probably saw 200 children.”

Police in Mount Horeb said they could not immediately provide more information. A person who answered the phone at the school district office declined comment.

A representative for the Dane County Sheriff’s Office could not immediately be reached.

The southwestern Wisconsin city is about 25 miles (40km) west of the state capital of Madison.

The district said in Facebook posts that it was preparing buses to bring students from schools to a place where families could reunite with their children.